1. Field of the Invention
The present invention relates to an active filter circuit. More specifically, the present invention relates to an active filter circuit suitable for an integration circuit (IC).
2. Description of the Prior Art
In a case where a filter is to be incorporated in an integrated circuit, due to variations of elements in the IC, a fluctuation takes place in a filter characteristic. Conventionally, in order to compensate for such a fluctuation, there was one method in which a dummy reference filter is provided in the IC and a reference signal is input thereto, and a feed-back control is applied to the reference filter so that a value of an output of the reference filter becomes a desired value. Based upon an idea that since a filter to be controlled comprises resistors and capacitors being the same as or correlative to that of the reference filter and therefore, a filter characteristic of the filter to be controlled can be adjusted to the same as or correlative to the characteristic of the reference filter if and when the same or correlative control signal is applied thereto, this method is a method for controlling the other filter to be controlled within the IC.
A prior art shown in FIG. 1 is one example of a case where a cut-off frequency fc of a lowpass filter is automatically controlled. First, a reference signal having a predetermined reference frequency Fref is input to a reference filter, and a level of a signal that is obtained by amplifying an output of the reference filter is compared with an output level of a reference frequency signal oscillator, a control voltage Vc by which both the levels are become coincident with each other is out, and the control voltage is applied to a variable capacitance diode Cv so that the filter characteristic is changed. By applying this feed-back control, when an attenuation amount at the reference frequency in the filter becomes equal to a gain of the amplifier, both levels becomes equal to each other and thus the filter is brought into an adjusted state.
In the conventional method shown in FIG. 1, it is not necessary to apply a signal externally to adjust the filter incorporated in the IC; however, since a dummy reference filter is to be provided in the IC, not only the number of elements in the IC increases but also the reference signal oscillator is needed.
One example of a filter circuit suitable for an IC is disclosed in, for example, Japanese Patent Publication No. 61-55860 published on Nov. 29, 1986.
In a filter circuit disclosed in Japanese Patent Publication No. 61-55806, in order to implement a high-pass filter circuit, it is necessary to add a subtracter, and therefore, there was a problem that not only the number of circuit components increases but also a circuit configuration becomes complex.